This invention relates to a method of recording a television signal on a record carrier tape.
The broad frequency band of television signals requires a high relative speed between the tape on which the signal is recorded and the recording head which performs the recording, usually a magnetic tape and a magnetic recording head. For this reason methods of recording television signals have become known in which the signal is recorded on a relatively slowly moving magnetic tape by a scanning device having a plurality of recording and playback heads which rotate at a relatively high speed in a plane non-parallel to the direction of tape motion. The heads are carried at the periphery of a rotatable part of the scanning device which also includes guide means for guiding the tape in the correct manner past the heads. The rotatable part of the scanning device is commonly termed a head wheel in the art and the same term is used in the present description and claims, although it is to be understood that the rotatable part of the scanning device may not in face resemble a wheel but may take any suitable form.
By using such methods the signal is recorded in a plurality of substantially parallel tracks which extend at an angle to the longitudinal direction of the tape. These known methods include some in which the information content of a complete television picture is distributed among several successive tracks. Thus, for example, in the so-called cross-track recording technique, in which four heads are arranged on a head wheel perpendicular to the direction of tape motion, each track contains the information content of 16 to 17 television lines. On the other hand, in a common inclined track recording technique, in which the tape loops the scanning device in a helical manner, each track contains the information content of approximately 50 lines.
The common factor in these known methods is that after each complete television picture has been recorded a whole number of revolutions of the head wheel has taken place--in other words at the start of recording each complete television picture the head wheel occupies the same angular starting position. The reason for this is that the development of these methods was based on the desirability for each complete picture to be recorded under the same conditions as regards the distribution of the line signals among the tracks and the angular position of the head wheel.
If, however, we divide the number of lines in a complete picture--e.g. 625 lines in the European television system and 525 lines in the U.S. television system--by the number of tracks per picture which can in practice be obtained given the requirement for an integral number of revolutions of the head wheel per complete picture and taking into account practical constraints resulting from the head wheel speed and geometrical considerations (e.g. diffusion breadth, gap width, track length), then with the known methods of recording mentioned above we do not obtain a whole number of lines per track. The result of this is that the beginning of the first complete line of each track--and thus also the beginning of each further line of the track--varies from track to track relative to the edge of the tape. Since it is also advantageous to switch from one head to another within one line, there is a residue of unused area on both sides of the tape area allocated for the video tracks--in other words the tracks are one line longer than is actually necessary.